1. Field of the Invention
This invention relates to a color filter for a liquid crystal display device, and to a liquid crystal display device which is provided with the color filter. In particular, this invention relates to a color filter for a liquid crystal display device, which is excellent not only in contrast but also in oblique visibility and to a liquid crystal display device which is provided with such features.
2. Description of the Related Art
In recent years, thin display devices such as a liquid crystal display device are increasingly demanded to enhance the picture quality and power-saving thereof and to reduce the manufacturing cost thereof. Therefore, in the case of the color filter to be employed in such display devices, it is desired to employ a photosensitive resin composition which is sufficiently high in color purity, in brightness and in contrast and which makes it possible to realize accurate patterning works of color layers having fine patterns, of a spacing layer, of a space controlling layer for controlling cell gap, and of a retardation layer at a low cost while developing the birefringence in conformity with various kinds of liquid crystal display modes.
Especially, in the case of a large television or a high picture quality monitor, where the display contrast thereof is not less than 2000, they are now demanded to exhibit not only a high front face contrast but also a very high level of display quality with respect to wide viewing angle characteristics including oblique viewing direction. It has been found out, as a result of optical designing of a liquid crystal display device as a whole, that it is difficult to completely correct the retardation of color filter, thereby inevitably leaving a small degree of retardation (for example, +10 nm or so) as a value of retardation in the thickness direction of at least one of the layers including the color layers, a spacing layer, a space controlling layer for controlling cell gaps and a retardation layer, thus resulting in the deterioration of oblique visibility.
In order to cope with this problem, various methods have been tried to reduce the quantity of retardation that the color filter may exhibit, the methods including one wherein a macromolecule having a planar structural group on its side chain is introduced into a color layer, or one wherein a birefringence-reducing particles having a double refraction index which is opposite in sign to that of a macromolecule is introduced into a color layer (see for example, JP-A 2000-136253 and JP-A 2000-187114).
Further, there has been proposed an idea to incorporate a retardation-adjusting agent in the color layers of color filter, thus enabling each of subpixels to have a different retardation, thereby making it possible to enable the viewing angle compensation of darkened state of a liquid crystal display device to be effected in the wavelength of almost all visible light zone without necessitating the provision of a polymeric liquid crystal layer in addition to the color layers or without necessitating the change of thickness in each of subpixels (see for example, JP-A 2008-20905, JP-A 2008-40486) and JP-A 2008-145868).
Furthermore, there has been proposed a method of improving the oblique visibility, wherein the values of retardation in thickness direction of the red, green and blue color pixels of color filter (these color pixels are herein defined as Rth(R), Rth(G) and Rth(B), respectively) are regulated, in conformity with the wavelength dispersibility of liquid crystal materials or of retardation films, to Rth(R)>Rth(G)>Rth(B) or Rth(R)<Rth(G)<Rth(B), thereby improving the oblique visibility (see for example, JP-A 2007-212603 and JP-A 5-196930).
However, it has been found out by the present inventors that the value of retardation in a thickness direction which the color layer may exhibit greatly differs depending on the kinds of pigment to be employed and that the magnitude of the value of retardation in a thickness direction becomes larger depending on the pulverization or dispersion of the pigment or on the kinds of matrix resin (for example, acrylic resin, cardo resin, etc.). Therefore, in the case of the conventional methods where a macromolecule having a planar structural group on its side chain is introduced into a color layer, or where a birefringence-reducing particles is introduced into a color layer, even if these methods are effective in increasing the value of retardation in a thickness direction in the positive direction or in decreasing a large positive value of retardation in a thickness direction within a positive range, these methods are not sufficiently effect in shifting the value of retardation in a thickness direction from zero in the negative direction within a small range, thus failing to solve the aforementioned problems.
Further, it has been found difficult to uniformly confine all of the retardations of red, green and blue color pixels to be employed in a color filter to a region ranging from zero to a small negative value while retaining a high contrast of color filter.
Since a styrene-containing polymer composition is excellent in transparency and in heat resistance, it is possible to use it as a color layer of color filter to be employed, for example, in a liquid crystal display, an electronic paper display, an electroluminescence panel, etc., or use it as material for forming a spacing layer. Further, the styrene-containing polymer composition can be modified to exhibit developing properties or photo-curing properties by copolymerizing it with an alkali-soluble monomer such as aromatic/non-aromatic polyvalent carboxylic acid-containing monomers or with a polymeric bonding-containing monomer, thus providing it with excellent functions as a polymeric composition for use as an electronic material.
On the other hand, the aforementioned styrene-containing polymer composition is known to have a negative birefringence (see for example, Development of Negative Birefringence Polymeric Material for Novel Wide Viewing Angle Retardation Films. TOSOH Research & Technology Review Vol. 48 (2004)) and hence has been suitably employed as a material for a negative birefringence retardation film. Therefore, it is expected that the aforementioned problems can be overcome by the incorporation of the styrene-containing polymer composition as a birefringence-adjusting agent in a material to be used for forming at least one layer selected from the color layers, the spacing layer, the space controlling layer for controlling cell gap, and the retardation layer of color filter for liquid crystal display. However, since the retardation of the color filter is relatively small as compared with that of other components to be used in a liquid crystal display device, no one has tried to employ the styrene-containing polymer composition up to date. Namely, there have been reported almost no studies which are dealt with a method of reducing the value of retardation in a thickness direction by the incorporation of a styrene-containing polymer composition into an alkali-developing photosensitive resin composition, for example.
Additionally, there is a problem that when a large quantity of the conventional styrene-containing polymer composition is incorporated into an alkali-developing photosensitive resin composition, storage stability of the photosensitive resin composition is deteriorated. Furthermore, the incorporation of styrene-containing polymer composition may lead to various problems including the deterioration of the alkali-developing properties of the photosensitive resin composition, the prolongation of developing time due to difficulties of suitably adjusting the developing rate, and susceptibility to peeling of a coated film from a substrate due to too fast developing rate on the contrary. Therefore, there is a limit in mixing ratio of the styrene-containing polymer composition, thus making it impossible to enable the styrene-containing polymer composition to sufficiently exhibit the effects thereof as a birefringence-adjusting agent.